1. Field of the Invention
The present invention relates to a color liquid crystal display apparatus and a method for producing the same. In particular, the present invention relates to a color liquid crystal display apparatus including liquid crystal molecules which are axially symmetrically aligned in liquid crystal regions partitioned by a polymer wall, and a method for producing the same.
2. Description of the Related Art
In the past, as a display apparatus utilizing an electrooptical effect, a twisted nematic (TN) liquid crystal display apparatus and super-twisted nematic (STN) liquid crystal display apparatus using nematic liquid crystal have been used. A technique of widening the viewing angle of these liquid crystal display apparatuses have been vigorously developed.
As one technique of widening the viewing angle of a TN liquid crystal display apparatus, Japanese Laid-Open Publication Nos. 6-301015 and 7-120728 disclose a liquid crystal display apparatus including liquid crystal molecules which are axially symmetrically aligned in liquid crystal regions partitioned by a polymer wall (i.e., an axially symmetrically aligned microcell (ASM) mode liquid crystal display apparatus). A liquid crystal region substantially surrounded by a polymer wall is typically formed on a pixel basis. In the ASM mode liquid crystal display apparatus, liquid crystal molecules are axially symmetrically aligned, so that the contrast is not changed even when an observer sees the apparatus in any direction. That is, the ASM mode liquid crystal display apparatus has wide viewing angle characteristics.
The ASM mode liquid crystal display apparatuses disclosed in the above-mentioned publications are produced by phase-separating a mixture of a polymerizable material and a liquid crystal material while inducing polymerization.
Referring to FIGS. 10A through 10I, a method for producing a conventional ASM mode liquid crystal display apparatus will be described. First, a glass substrate 908 on one side of which a color filter and electrodes are formed (FIG. 10A). For simplicity, the electrodes and the color filter formed on the glass substrate 908 are not shown. A method for forming the color filter will be described later.
Next, a polymer wall 917 for axially symmetrically aligning liquid crystal molecules is formed, for example, in a lattice shape on the surface of the glass substrate 908 on which the electrodes and the color filter are formed (FIG. 10B) in the following manner. A photosensitive resin material is spin-coated on the surface of the glass substrate 908, and the resultant substrate 908 is exposed to light through a photomask, followed by development, whereby a lattice-shaped polymer wall is formed. The photosensitive resin material may be negative or positive. Although the step of separately forming a resist film is added, the resist film can be formed by using a resin material having no photosensitivity.
A column-shaped projection 920 is patterned on a part of the top of the polymer wall 917 (FIG. 10C). The column-shaped projections 920 are also formed by exposing a photosensitive color resin material to light, followed by development.
The surface of the glass substrate 908 on which the polymer wall 917 and the column-shaped projections 920 are formed is covered with a vertical alignment agent 921 such as polyimide (FIG. 10D). On the other hand, a counter glass substrate 902 on which electrodes are formed is also covered with the vertical alignment agent 921 (FIGS. 10E and 10F).
Two substrates thus obtained are attached to each other so that the surfaces having electrodes face each other, whereby a liquid crystal cell is formed (FIG. 10G). A distance (cell gap: thickness of a liquid crystal layer) between two substrates is defined by the sum of the heights of the polymer wall 917 and the column-shaped projection 920.
A liquid crystal material is injected into the gap of the liquid crystal cell by a vacuum injection method or the like (FIG. 10H). Finally, for example, by applying a voltage across the electrodes on both substrates, liquid crystal molecules are axially symmetrically aligned in a liquid crystal region 915 (FIG. 10I). The liquid crystal molecules in the liquid crystal region 915 partitioned by the polymer wall 917 are axially symmetrically aligned with respect to an axis 916 (vertical to both substrates) represented by a broken line in FIG. 10I.
FIG. 11 shows a cross-sectional structure of a conventional color filter. A black matrix (BM) for blocking light in a gap of a coloring pattern and coloring resin layers of red (R), green (G), and blue (B) corresponding to each pixel are formed on a glass substrate. An overcoat (OC) layer (thickness: about 0.5 to about 2.0 .mu.m) made of acrylic resin, epoxy resin, or the like is formed for the purpose of improving flatness. An indium tin oxide (ITO) film of a transparent signal electrode is formed on the OC layer. The BM is generally made of a chromium film (thickness: about 100 to about 150 nm). The coloring resin layer is made of a resin material colored with a dye, a pigment, or the like, and the thickness thereof is generally about 1 to about 3 .mu.m.
The color filter is formed by patterning the photosensitive coloring resin layers formed on the substrate by photolithography. For example, photosensitive coloring resin materials of R, G, and B are formed, exposed to light, and developed (three times in total), whereby a color filter of R, G, and B can be formed. As a method for forming the photosensitive coloring resin layer, there are a method for coating a liquid photosensitive coloring resin material (diluted in a solvent) on a substrate by spin-coating and a method for transcribing a dry-filmed photosensitive coloring resin material onto a substrate. The above-mentioned ASM mode liquid crystal display apparatus is produced by using the color filter thus formed, whereby a color liquid crystal display apparatus having wide viewing angle characteristics can be obtained.
However, in the case where the above-mentioned ASM mode liquid crystal display apparatus and the method for producing the same are applied to a color liquid crystal display apparatus, the following problems arise.
As described above, in the past, a coloring resin layer of each color for a color filter is formed, an OC layer is formed thereon, and thereafter, an ITO film is formed and patterned. The OC layer is provided for the purpose of flattening nonuniform portions of the surface of the color filter (boundary portions where coloring resin layers of R, G, and B are adjacent to each other, i.e., an overlapped portion of each coloring resin layer and a black matrix), preventing disconnection of the ITO film in the course of production, and preventing each coloring resin layer of R, G, and B from being corroded by an etchant (e.g., aqua regia, ferric chloride) of ITO in the step of etching the ITO film for patterning.
However, the OC layer itself slightly absorb light, which decreases transmittance, resulting in decreased brightness. Furthermore, when a polymer wall is formed on electrodes, the voltage substantially applied to a liquid crystal layer is decreased due to the capacity of the polymer wall. Furthermore, the retention ratio of the voltage (hereinafter, referred to as a voltage retention ratio) applied to the liquid crystal layer is decreased. Furthermore, it was found that the decrease in the voltage retention ratio likely causes image sticking during display, which is a problem in terms of reliability.